The invention relates to the field of compositions for the treatment of metal surfaces, intended to provide both a prelubricating effect and a temporary corrosion protection effect.
The prelubricating effect is characterized by an improvement in the tribological properties of the surface treated in this way and then oiled in a conventional manner; this effect is beneficial for facilitating the forming operations, for example drawing operations.
The corrosion protection effect is temporary in the sense that a simple cleaning of the treated surface allows the treatment layer to be removed; this ability to be cleaned is important, for example when preparing to paint the surface.
These metal surface treatments are generally carried out in two steps:
application of a liquid homogeneous layer of the treatment composition on the surface;
drying of the layer applied, suitable for removing almost all of the liquid water from this layer, at a low enough temperature to avoid any deterioration of the treatment components.
The so-called xe2x80x9cdryxe2x80x9d film that is obtained may then also contain large amounts of water included in the composition of the film (as water of hydration, for example).
Compositions are sought which can be applied homogeneously as thin films and are effective with a low surface density; the desired surface density, measured in the dry state, is then generally less than 2.5 g/m2.
Conventionally, these surface treatment compositions comprise, as essential components in solution or in emulsion, at least one sufficiently film-forming binder, at least one lubricant and at least one surfactant for promoting film spread over the surface to be treated.
In the case of an emulsion, the emulsifiers used generally have a surfactant effect promoting film spread.
These treatment compositions generally furthermore comprise corrosion inhibitors which may also have a surfactant effect.
The invention relates more particularly to the field of compositions for treating a metal surface, which comprise a binder based on pregelatinized starch and are intended to form a homogeneous dry film on this surface.
Pregelatinization of the starch is a specific operation, carried out by the producer, which destroys the crystalline structure of the starch granule and reduces the hydrogen bonds responsible for this structure. This operation is carried out, at approximately 100xc2x0 C., by rolling or extension. Pregelatinized starch is then obtained which is in the form of a powder. It is this operation, known per se, which allows the (partially gelatinized) starch to be soluble in cold water and allows low-viscosity solutions to be obtained. This pregelatinization operation will make it possible to control (particularly with regard to viscosity) the phenomena of gelatinization or gelling in the presence of water.
Starch is essentially a homopolymer of D-glucose (highly hydroxylated glucosyl monomer) composed mainly of a mixture of two types of polymer:
amylose, an essentially linear molecule, capable of complexing hydrophobic molecules such as fatty acids, able to react with emulsifiers and known for its film-forming propertiesxe2x80x94T(melting)≈120xc2x0 C.;
amylopectin, an essentially branched molecule having a very high molecular mass (107 to 108)xe2x80x94T(melting)≈45xc2x0 C.
The amylose content of the starch depends on its plant origin, as indicated in Table I: certain genetically modified varieties may give starches richer in amylose.
The binding effect of the pregelatinized starch results from chemical mechanisms described below.
The water absorption by the starch grains is manifested by a reversible swelling of the amorphous part and the formation, firstly, of interchain hydrogen bonds; secondly, the water acts as a plasticizer which allows alignment of the crystallites by extension of the inter-crystalline amorphous phases and a large part of the absorbed water is then bound to the structure: at least 30% water is necessary for the plasticization to occur; the plasticization lowers the glass transition temperature (of the amorphous part).
A pregelatinized starch gel can then be described as a discontinuous phase composed of swollen starch granules enriched with amylopectin dispersed in a continuous phase of dissolved amylose.
Among the various products coming from the pregelatinization of starches, there are, among others:
fluid starches;
crosslinked starches;
dextrins, which are linear or branched oligosaccharides generally containing from 5 to 10 glucosyl units;
starch esters, such as starch acetates or phosphates;
starch ethers, such as O-alkyl starch ethers.
To achieve these various products coming from the gelatinization of starches, various modification means are used, such as xe2x80x9cenzymaticxe2x80x9d modifications, xe2x80x9cdepolymerizationxe2x80x9d modifications and chemical xe2x80x9ccrosslinkingxe2x80x9d, xe2x80x9csubstitutionxe2x80x9d, xe2x80x9cdextrinizationxe2x80x9d or xe2x80x9csaccharificationxe2x80x9d modifications.
Document FR 2,508,051 (ROQUETTES) describes (see especially claim 13) a surface treatment composition, especially for temporary corrosion protection, comprising, with respect to the dry matter:
from 30 to 95% of an extruded waxy starch;
from 1 to 25% of a plasticizer;
from 0.5 to 5% of a surfactant or wetting agent.
According to that document (page 5, lines 17-32), the plasticizer added to the starch-based composition makes it possible to prevent crazing and flaking of the film resulting from the treatment; this plasticizer is chosen from the group comprising not only sorbitol, a polyethylene glycol or glycerol, but also glucose syrups, lactates, gluconates, emulsifiable waxes, urea, thiourea and nitrates.
According to that document (page 6, lines 2 et seq), for effective temporary protection in wet environments, the treatment composition must also contain a resin or an insolubilising or crosslinking agent, generally 2 to 30%, such as resorcinol-formaldehyde or cyanamid-formaldehyde; consequently, the film resulting from the treatment is at least partially crosslinked, thereby impairing its ability to be cleaned.
According to that document (page 7, lines 27-28), the surface treatment composition may also contain anti-rust agents, that is to say a corrosion inhibitor.
That document describes the application of this composition to steel surfaces (example 1), but, in order to obtain effective corrosion protection, the surface density to be applied must be very high, namely 100 g/m2 on page 16 and 200 g/m2 on page 19.
The object of the invention is to provide a surface treatment composition which is effective at much lower surface densities, both for temporary corrosion protection and for prelubrication; the object of the invention is to provide an easily cleanable surface treatment composition.
For this purpose, the subject of the invention is an easily cleanable aqueous composition for treating a metal, especially steel, surface, characterized in that it comprises, as an emulsion, at least 2% by weight of pregelatinized starch and at least 1% of vegetable wax or oil, or at least 1% of synthetic oil or wax of equivalent chemical nature.
Using specifically an emulsified plant-based oil or wax in the treatment composition based on pregelatinized starch it is thus possible, according to the invention, to obtain both effective corrosion protection in a wet atmosphere and a prelubricating effect, even if this composition is applied as a thin film and without a crosslinking agent, so as to be easily cleanable.
The invention may also have one or more of the following characteristics:
the said starch is crosslinked;
the said starch is a potato or maize starch;
the amylose content of the said starch is less than or equal to 25%;
the pH of the composition is between 6.5 and 8;
the weight content of pregelatinized starch is less than or equal to 7% and the weight content of oil or wax in the said composition is less than or equal to 5%;
the said vegetable wax or oil is chosen from the group comprising carnauba wax, jojoba oil, candelilla wax and mixtures thereof;
the composition also comprises at least one corrosion inhibitor, preferably chosen from the group comprising an alkylamine carboxylate, a hydroethyl-alkylene-imidazoline and mixtures thereof;
the said corrosion inhibitor is a mixture of an alkylamine carboxylate and a hydrotheyl-alkylene-imidazoline, preferably in a proportion of approximately 2:1.
The subject of the invention is also a process for treating a metal surface using a composition according to the invention, comprising the steps consisting in applying a layer of the said composition to the said surface and then in drying the said layer so as to obtain a dry film, characterized in that the surface density of the said dry film is between 1 and 2.5 g/m2.
Preferably, the said metal surface is a steel surface.
Finally, the subject of the invention is the use of this process for the temporary corrosion protection and/or prelubrication of the said surface.
The invention will be more clearly understood on reading the description which follows, given by way of non-limiting example.
The surface treatment compositions according to the invention are prepared in the following manner:
dissolving pregelatinized starch in water;
independently, using suitable emulsifiers, such as ethoxylated fatty alcohols, emulsifying the plant-based wax or oil in water;
mixing the pregelatinized starch solution with the aqueous emulsion obtained;
where appropriate, adding other components, such as corrosion inhibitors, to the mixture.
As plant-based wax or oil, it is preferred to use carnauba wax, jojoba oil or candelilla wax.
The carnauba wax comes from a Corypha Cerifera palm; it is essentially composed of a triglyceride of cerotic acid; the melting point of this triglyceride is very high, namely 80 to 85xc2x0 C.; cerotic acid is a C26 saturated fatty acid.
Jojoba oil is a liquid wax contained in a high proportion (45 to 60%) in the seeds of a Simmondsia Chinensis shrub which grows naturally in semi-desert regions (North Mexico, Arizona, Texas); it consists of aliphatic esters based on mono unsaturated fatty alcohols and fatty acids, mainly comprising eicosenoate of eicosenol and docosenol (C19H37COOxe2x80x94C20H39, C19H37COOxe2x80x94C22H43); its melting point is between 6 and 7xc2x0 C.; this oil does not easily become rancid.
Candelilla wax comes from Euphorbia Cerifera and Pedilantus Pavonis shrubs which grow naturally in the north of Mexico and in the south of Texas; approximately half of it consists of hydrocarbons, mainly hentricontane C31H64 and the other half consists of a mixture of fatty acids, of aliphatic and triterpene alcohols and of their esters; its melting point is between 60 and 70xc2x0 C.
The surface treatment compositions are prepared so as to obtain the following weight proportions of the main components:
2 to 7% pregelatinized starch;
1 to 5% oily or waxy phase.
Preferably, corrosion inhibitors are also added to these compositions: preferably an alkylamine carboxylate and/or a hydroethyl-alkylene-imidazoline are used; hydroethyl-alkylene-imidazolines are described in document EP 651,074 (PETROLITE).
In this case, the proportions by weight of these inhibitors in the treatment composition are preferably as follows:
0.5 to 3% alkylamine carboxylate;
0.25% to 2% hydroethyl-alkylene-imidazoline.
Within the compositional ranges defined above, the proportions of binder, lubricant and, where appropriate, inhibitor are adapted, in a manner known per se, in order to obtain, after treatment, both the best tribological properties and the best corrosion protection.
In order to treat the metal surface, the composition prepared is then applied to this surface so as to form a liquid homogeneous layer which is then dried in order to remove almost all of the liquid water in this layer, at a low enough temperature to avoid any deterioration of the treatment components, especially of the pregelatinized starch.
In the so-called xe2x80x9cdryxe2x80x9d state, the temporary protective film thus obtained contains large amounts of water included in the composition of the film, especially in the pregelatinized starch.
Preferably, the application conditions are adapted so as to obtain a dry film having a surface density of between 1 and 2.5 g/m2 approximately.
Below 1 g/m2, it is difficult to obtain a film with a sufficiently uniform thickness and the corrosion protection is not always sufficiently well guaranteed.
Above 2.5 g/m2, the film runs the risk of being more difficult to clean; there is also a risk of annoying occurrences of sticking when treated surfaces are stacked.
Thus, by using the composition according to the invention for treating bare steel surfaces, very good temporary corrosion protection and substantial improvement in tribological properties are obtained simultaneously; the dry treatment film is easy to clean.
In addition, the lubricator properties of the dry treatment film obtained using the composition according to the invention are maintained, even after being liberally sprayed with an aqueous oil emulsion of the type used for lubrication during drawing.
Finally, since the essential products of this treatment composition and of the dry film obtained after treatment are natural and biodegradable, this results in a significant advantage from the environmental standpoint.
Further advantages of the process of the invention will appear on reading the examples of the present invention which are given below without implying any limitation.
MATERIALS:
For the examples illustrating the invention, the treatment composition contains, unless otherwise indicated in the examples below:
1xe2x80x94as starch: pregelatinized potato starch, reference LAB 2431 or Pregeflo PJ20, which are sold by the company ROQUETTE; these starches differ by their particle size; both are modified by crosslinking; they are both soluble in water at room temperature in the proportions used for carrying out the invention;
2xe2x80x94as vegetable wax or oil, carnauba wax: a self-emulsifiable wax with the reference Cerax M33 or Cerax AO28, these being sold by the company BARLOCHER France; the melting point of the AO28 wax is between 80 and 85xc2x0 C. while that of the M33 wax is between 62 and 67xc2x0 C.; the low melting point of the M33 wax is explained by the presence, in this commercial product of approximately 10% of emulsifiers based on ethoxylated fatty alcohols and approximately 2 to 5% paraffin; these additives improve the conditioning of the wax in solid granule form and make it easier to emulsify it.
In the tests described below, unless otherwise indicated, the treatment compositions are applied to sheets of bare steel so as to form a 1 to 2 g/m2 dry film.
METHODS:
1xe2x80x94xe2x80x9cHot-wetxe2x80x9d Corrosion Test (or xe2x80x9cFKWxe2x80x9d):
The coupons to be tested are placed as they are in an environmental chamber, corresponding to the DIN 50017 standard and simulating the corrosion conditions of an outer turn of a coil of sheet metal or of a cut metal sheet during storage.
The environment cycle to which the coupon to be tested is subjected is as follows: 8 h at 40xc2x0 C. and 95 to 100% relative humidityxe2x80x9416 h at 20xc2x0 C. and 75% relative humidity.
The result of the test is obtained by noting the number of successive cycles before traces of corrosion appear on the coupon.
2xe2x80x94Friction or Lubrication Test:
For these tests, a plane-plane tribometer of conventional type is used.
Before the measurement, the pieces to be tested, already treated on the surface as the case may be, are oiled using a whole oil, reference 8021 from the company QUAKER; the grammage of oil deposited is about 2 g/m2.
The oiled test pieces are then clamped with a clamping force FS between two small high-speed steel plates having an area 1 cm2.
The friction coefficient k is measured while moving the test piece at a constant speed V with respect to the small plates over a total distance D of 180 mm, while progressively increasing the clamping force FS from 200 daN at the start of the test to 2000 daN at the end of the test.
The pull speed V is 10 mm/s, unless otherwise specified.
The curve showing the variation in friction coefficient k as a function of time or of the clamping force FS is generally a decreasing curve, but more rarely a constant curve; in order to evaluate the tribological performance, the friction coefficient is generally measured at the end of travel, for Fs≈1800 daN.
3xe2x80x94Test for Compatibility with the Soluble Oils:
This is a test for checking that the lubricating properties of the dry treatment film obtained using the composition according to the invention are maintained, even after liberally spraying it with an aqueous oil emulsion of the type used for lubrication during drawing.
These aqueous emulsions or solutions are used to lubricate but also to cool the drawing tools, especially when the rates are high; it is therefore necessary to check that, by liberally dousing a surface treated according to the invention with an aqueous drawing emulsion or solution, the tribological properties of the dry film resulting from the treatment do not deteriorate, for example by leaching or by forming soaps, to the point of even causing seizure.
For this test, the soluble oil referenced 72CC from the company QUAKER is used, the oil being diluted to 10% with water, and then the friction measurement corresponding to Test 2 above is carried out directly, without reoiling.
The friction measurement therefore gives an indication of the retention or lack of retention of the lubricating properties provided by the dry film.
4xe2x80x94Cleanability Test:
Water at 50-60xc2x0 C. and at 1 to 3xc3x97105 Pa is sprayed for 1 to 3 minutes onto a treated specimen provided with a dry film.
After this cleaning operation, the treated surface is examined; disappearance of the dry film means good cleanability.